1. Field of the Invention
The present invention relates to fiber optic cables, particularly all dielectric self-supporting (ADSS) fiber optic cables intended for installation over long spans.
2. Discussion of the Known Art
ADSS fiber optic cables are constructed to be self-supporting between poles, towers, or other fixed structures that support existing high voltage utility power lines. The cables contain optical fibers that are typically used for networked communication services. See, e.g., IEEE Standard 1222-2011, at page 7. The fibers may be routed loosely inside flexible buffer tubes contained within the cable (so called “loose tube” cables), or the fibers may be arranged in a ribbon configuration. The present invention relates to loose tube ADSS cables.
Once ADSS cables are installed and any co-located power lines are energized, the cables can be exposed to electric fields as high as 25 kV/meter or more. Thus, the cables must be made of only non-conductive or dielectric materials to avoid electrical arcing, heating, or other damage from the intense fields.
The National Electric Safety Code (NESC) provides mandatory load bearing requirements for overhead lines of all kinds, including power transmission lines, power distribution lines, and both wire and fiber optic telephone and cable TV lines including service drops to houses and commercial buildings. To account for the icing and wind loads that overhead lines must sustain throughout the year depending on their location, the NESC provides ice- and wind-load maps in which the United States is divided into three districts for ice loads: namely, Heavy, Medium, and Light districts.
In the Heavy ice-load district, encompassing states in the northeast and north central regions of the U.S., the uniform ice thickness is 0.5 in. In the Medium and Light districts toward the south, the uniform ice thicknesses are 0.25 in. and 0 in., respectively. The minimum wind load required for these districts is 4, 4, and 9 lb/ft2, respectively. Also, a factor as high as 1.5 is applied to the basic ice loads for major transmission lines. Some northern utilities may develop even more stringent requirements if ice loads occur more frequently and to a higher degree than the NESC requirements anticipate for their location.
In addition to having to meet the applicable NESC ice and wind load requirements, when ADSS fiber optic cables are prepared for installation over long spans, (for example, between approximately 500 and 1050 feet in a NESC Heavy load district, between approximately 800 and 1650 feet in a Medium load district, and between approximately 1050 and 2200 feet in a Light load district), the cables are subjected to high compression or crushing forces as, they are rolled over sheaves, tensioners, or blocks. Clamps that are used to attach the cables permanently on the supporting structures also apply large crushing forces on the cables. To protect the fragile optical fibers inside the buffer tubes within the cables, the tubes must be able to resist deformation whenever large forces are applied to the cables both during and after installation. If such forces make the buffer tubes deform or flatten into a non-circular cross section, the environment of the fibers inside the tubes is physically altered and their ability to move freely whenever the cable is stressed becomes limited. Yet, buffer tube deformation continues to be a common failure mode for long span ADSS fiber optic cables, particularly during the installation phase.
In traditional ADSS cables, the buffer tubes are made of poly (butylene terephthalate) or PBT which has high crush resistance. Also, the tubes are often gel filled so that the combination of the high-modulus PBT buffer tubes with an incompressible gel filling prevents the tubes from deforming significantly during and after typical cable installations. Notwithstanding, the use of the gel filling presents two major problems, namely, cable weight and additional installation time.
First, PBT is a heavy engineering plastics material with a density of 1.3 gram/cc. The gel is also heavy and has a density in the range of 0.8 to 0.85 g/cc. Because of this, traditional ADSS cables require substantial internal reinforcement including expensive aramid yarns to act as dielectric strength members so that the cables are self-supporting over a rated span.
Second, providing a gel filling inside the buffer tubes is messy and costly. The gel is sticky and must be removed thoroughly before the optical fibers inside the tubes are exposed for splicing or termination in the field. It will therefore be understood that by eliminating the need for the gel filling, reducing the quantity of aramid yarn, and making the buffer tubes from a material that is lighter but harder than PBT, the cost of making a long span ADSS cable can be reduced and the cable installation made both simpler and safer.
FIG. 1 of the present application is a cross-sectional profile of a loose tube ADSS cable 10 sold by OFS Fitel, LLC, under the trademark PowerGuide® Short Span DT. The cable is intended for light duty, short span, non-custom applications. As seen in the drawing, the cable 10 includes:
A central strength or tension rod member 12 made of epoxy-fiberglass;
Buffer tubes 14 stranded about the central rod member 12 with a reverse oscillating lay (ROL) twist;
Optical fibers 16 routed inside the buffer tubes 14;
A water swellable yarn 18 or other water blocking material routed with the fibers inside each tube 14;
Dry water blocking materials 20 applied over the tubes 14;
Dielectric strength members 22; and
A medium-density polyethylene (MDPE) outer jacket 24.
For applications where the cable 10 is exposed to electric fields higher than about 12 kV/m, the outer jacket 24 can be made of a track resistant HDPE compound such as Borstar® HE6081 to prevent electrical arcing and tracking in the jacket. The jacket 22 may also contain a filler to make the surface of the jacket hard to wet by reducing surface energy, and to resist ablation of the jacket from electric charge.
Each buffer tube 14 is made of a polypropylene (PP) material having an elastic modulus greater than 1420, for example, Basell EBS 777D which has an elastic modulus of 2110 and is similar to that of PBT which is typically quoted at 2300-2600 MPa. The modulus of the buffer tubes 14 is also higher than that of typical commercial impact-modified PP materials which have a modulus of only about 1420 MPa and have been used to make buffer tubes for ADSS cables. See Table 1, below. Moreover, the density of PP is only about 0.92 g/cc, while PBT has a density of 1.3 g/cc.
The high modulus PP buffer tubes 14 allow the cable 10 to resist deformation from crushing loads at least to the same extent as traditional ADSS cables, and without the need for a gel filling which has a density typically in the range of 0.8-0.85 g/cc. Instead of using a gel to prevent water penetration in the cable 10, a light water swellable yarn 18 may be routed together with the fibers in each buffer tube 14. See, e.g., U.S. Pat. No. 7,099,542 (Aug. 29, 2006), and U.S. Pat. No. 5,630,003 (May 13, 1997), both of which are incorporated fully by reference. Instead of the yarn 18, a water blocking powder may be pre-coated on the fibers 16.
The lighter buffer tubes 14 and the absence of a gel in the tubes allows the weight per unit length of the cable 10 to be reduced compared to gel-filled cables. Thus, the quantity of aramid yarn and other expensive strength members usually needed to support the cable 10 over a given span are also reduced.
TABLE 1Properties of PP ResinsBPExxon-DowBasell(Amoco)MobilBorealisJQDA-EBS 32407822NE7BB702W2221777DModulus, MPa14201420142021202110Melt Flow 5.04.04.04.85.3Rate, g/10 min
An early drawback of ADSS cables resided in the fact that because the cables are typically co-located with high voltage utility power lines, communications cables containing copper wire could not be lashed onto ADSS cables. Because the use of ADSS cables in distribution networks for Fiber-to-the-Home (FTTH) systems is increasing, there is no longer a great need to lash other cables over an installed ADSS cable. ADSS cables can now be used advantageously in leading-edge FTTH networks, with different cable configurations for trunk, feeder, and drop portions of the network. In addition, ADSS cable systems can be installed faster and more cost-effectively than similar lashed cable systems.
Notwithstanding the known art, there remains a need for a long span ADSS fiber optic cable capable of withstanding large crushing forces and stresses exerted on the cable during typical installation procedures, and after the cable is attached permanently to supporting structures that are spaced more than approximately 500 feet apart from one another in the NESC Heavy loading district.